1. Field of the Invention
This invention relates generally to wireless communication systems. More particularly, it relates to a wireless communication system using a plurality of antenna elements with weighting and combining techniques for optimizing antenna diversity and combining gain.
2. Description of the Related Art
Recently, the market for wireless communications has enjoyed tremendous growth. Wireless technology now reaches or is capable of reaching virtually every location on the face of the earth. Hundreds of millions of people exchange information every day using pagers, cellular telephones and other wireless communication products.
With the appearance of inexpensive, high-performance products based on the IEEE 802.11a/b/g Wireless Fidelity (Wi-Fi) standard, acceptance of wireless local area networks (WLANs) for home, Small Office Home Office (SOHO) and enterprise applications has increased significantly. IEEE 802.11b/g is a standard for a wireless, radio-based system. It operates in the unlicensed 2.4 GHz band at speeds up to 11 M bits/sec for IEEE 802.11b and 54 M bits/sec for IEEE 802.11g. The IEEE 802.11b/g specification sets up 11 channels within the 2.4 GHz to 2.4835 GHz frequency band which is the unlicensed band for industrial, scientific and medical (ISM) applications. IEEE 802.11a is another standard for a wireless, radio-based system in the ISM band. It operates in the unlicensed 5-GHz band at speeds up to 54 M bits/sec.
It has been found that WLANs often fall short of the expected operating range when actually deployed. For example, although a wireless Access Point (AP) is specified by a vendor as having an operating range of 300 feet, the actual operating range can vary widely depending on the operating environment.
In particular, WLAN performance can be greatly degraded by direct and multipath radio interference. Multipath occurs in wireless environments because the radio frequency (RF) signal transmitted by the subscriber is reflected from physical objects present in the environment such as buildings. As a result, it undergoes multiple reflections, refractions, diffusions and attenuations. The base station receives a sum of the distorted versions of the signal (collectively called multipath).
Similarly, in any indoor wireless system, multipath interference effects occur when the transmitted signal is reflected from objects such as walls, furniture, and other indoor objects. As a result of multipath, the signal can have multiple copies of itself, all of which arrive at the receiver at different moments in time. Thus, from the receiver's point of view, it receives multiple copies of the same signal with many different signal strengths or powers and propagation delays. The resultant combined signal can have significant fluctuation in power. This phenomenon is called fading.
Unlike all other parts of the radio spectrum, a license is not required to operate a transmitter in the ISM bands specified in IEEE 802.11 a/b/g. In exchange for this license-free environment, users implementing the IEEE 802.11 b/g and IEEE 802.11a standards are subject to interference from other users of the bands. The 2.4 to 2.4835 GHz ISM band is particularly sensitive to interference because it is populated with numerous wireless networking products such as Bluetooth systems, HomeRF systems, IEEE 802.11b WLAN devices, microwave ovens, and cordless phones that can result in significant interference. This interference is the result of a myriad of incompatible data transmission techniques, uncoordinated usage of spectrum, and over-subscription of the available spectrum.
Many devices operating in the 2.4 to 2.4835 GHz ISM band can either be classified as direct sequence spread spectrum (DSSS) or frequency hopping spread spectrum (FHSS) systems. The DSSS data transmission scheme is used primarily by IEEE 802.11b systems. FHSS systems, such as Bluetooth devices, differ from DSSS systems in their implementation for avoiding interference. FHSS systems avoid interference with other transmission signals in the same band by hopping over many different frequency channels. To provide FHSS systems with more bandwidth, the United States Federal Government Federal Communications Commission (FCC) has allowed FHSS systems to operate at wider bandwidths. The operation of FHSS systems at wider bandwidths has the potential to increase interference between DSSS and FHSS products. The interference level of narrowband FHSS systems on DSSS transmission has already been found to be severe.
There are additional elements of performance degradation in a network of 802.11b/g WLAN access points (APs). Since the 802.11b/g channel bandwidth is approximately 16 MHz, only three non-overlapping channels operating in proximity can be accommodated without interfering with one another. The channel re-use factor imposes a severe restriction on implementation of 802.11b/g based systems which requires significantly more effort in the network deployment, and increases the chances of interference and packet collision especially within an environment with a dense user cluster, such as in an office building.
Several approaches for improving the operating performance and range in a fading environment have been suggested. In one conventional approach, selection antenna diversity is used to reduce the effect of multipath fading. Multiple antennas are located in different locations or employ different polarizations. As long as the antennas have adequate separation in space or have a different polarization, the signal arriving at different antennas experiences independent fading. Each antenna can have a dedicated receiver or multiple antennas can share the same receiver. The receiver(s) checks to see which antenna has the best receiving signal quality and uses that antenna for the signal reception. The performance gain thus achieved is called diversity gain. The performance gain increases with the number of diversity antennas. The drawback of the selection diversity approach using a single shared receiver is that fast antenna switching and signal quality comparison is required. Since an 802.11(a, b, g) signal has a short signal preamble, only two diversity antennas are typically employed. This achieves a diversity gain of approximately 6 dB in a flat Rayleigh fading environment at the required frame error rate. The diversity gain decreases to 3 dB when delay spread is 50 ns and 0 dB when delay spread is 100 ns.
U.S. Pat. No. 6,115,762 describes an embedded antenna formed on a printed circuit board installed in a computing device. The antenna may include multiple radiating and receiving elements for mitigating multipath effects and/or responding to steering circuitry to form a directed antenna beam.
In another conventional approach, signal combining is used to provide improved performance in a fading environment. Signal combining techniques employ multiple spatially separated and/or orthogonally polarized antennas. The received signal is obtained by combining the signals from the multiple antennas. One technique for providing optimal signal quality is known as maximal ratio combining (MRC). To achieve the best signal quality, the received signal from each antenna is phase-shifted such that the resultant signals from all antennas are in phase. In addition, the signal from each antenna is scaled in amplitude based on the square root of its received signal-to-noise ratio. In an open loop implementation, a training sequence is transmitted first, followed by the signal containing information content. The received signal from each antenna is downconverted and passed through a channel filter. Separately, the received signals from all antenna elements are initially multiplied by a set of antenna weights (with arbitrary initial values) before they are combined into a single signal. The combined signal is then downconverted and passed through a channel filter. The antenna weights are a set of complex baseband signals and are derived by correlating received signal from each antenna element with the combined signal. Signal correlation is performed by multiplying the complex conjugate of the received signal from each antenna with the combined signal (called the reference signal) and passing it through a low pass filter (or integrator). Assuming that the averaged received noise power is the same in all received signals, the received signal envelope is proportional to the square root of the received signal-to-noise ratio. Under this condition, the magnitude of the antenna weight is proportional to the square root of the received signal-to-noise ratio. The generated set of antenna weights thus derived achieves the criterion for maximal ratio combining and the combined signal maximizes the received signal-to-noise ratio. Once the values are derived, the antenna weights are updated. The disadvantage of the open loop implementation is that the errors in antenna weights can accumulate over the signal processing steps. If there is any error in the derived antenna weights, the system can not detect the error automatically since there is no feedback mechanism. In addition, the system needs to detect the signal arrival and automatically activate the open loop antenna weight estimation process and update the antenna weight upon completion of the required steps, which increases the complexity of the system. Another disadvantage of the open loop implementation occurs when the initial set of antenna weights happen to produce a combined signal whose power is significantly lower than those from individual antenna. The antenna weight thus derived can thus have significant error due to the low signal level of the combined signal.
Another combining technique that maximizes the output signal-to-interference-plus-noise ratio is known as minimum mean square error (MMSE) combining. Signal combining techniques typically achieve better performance than the selection diversity antenna approach at the expense of added implementation complexity.
The signals can be combined with combining techniques based on a weighting scheme. Weights used in combining techniques can be generated with blind and nonblind techniques. In nonblind techniques, the received signal is demodulated and data sequences in the received signal are used to determine the portion that is the desired signal and the portion that is noise and interference. The demodulated signal is used to determine the combining weights through correlation with the received signals. In blind techniques, a property of the signal is used to distinguish it from interference and noise. In one approach, a constant modulus algorithm (CMA) is used to take advantage of a signal property of a constant signal envelope in order to generate a set of antenna weights such that the constant envelope property can be maintained. A shortcoming with nonblind techniques is that they require substantial modification of receiver application specific integrated circuit (ASIC) (baseband and media access controller (MAC), to use the demodulated signal. Thus, nonblind techniques generally must be incorporated into each vendor's WLAN integrated circuit (IC). Blind techniques do not use the demodulated signal and, therefore, can be added to the receiver with little or not modifications. Conventional blind techniques have the shortcoming that the receiver can generally not distinguish the desired signal from interference, and therefore the combiner weights can adapt to either the desired signal or to another wireless user's signal. Signal combining techniques typically achieve better performance than the selection diversity antenna approaches at the expense of added implementation complexity.
Another known approach to achieve performance improvement is through equalization, either in the time or frequency domain. In this technique, the multipaths arriving at the receiver are delayed, phase shifted, and amplitude scaled before they are combined (equalized). Equalization typically works better when the delay spread is large (>100 ns). The performance enhancement as a result of equalization adds to the diversity gain of antennas.
U.S. Pat. Nos. 4,736,460 and 4,797,950 describe a multipath reduction system including a CMA adaptive array. The CMA adaptive array includes at least two antenna elements and weighting means coupled to the antenna elements for selectively weighting the received signals by a selected weight factor. The weighted signals are added together to generate an adaptive array output signal. An envelope detector receives the adaptive array output signal for generating an amplitude envelope output signal of the adaptive array output signal. A multiplier receives the amplitude envelope output signal for generating a feedback signal. The feedback signal is received at CMA adaptive array for automatically redefining the weight factors based on the feedback signal and corresponding weighting means input signal. The use of CMA algorithm as described above has problems distinguishing the desired signal from the interfering signals.
Another conventional blind weight generation method is a power inversion technique, which generates weights that suppress the strongest received signals. This technique is acceptable when the interfering signals are much stronger than the desired signal. However, the desired signal can also be suppressed if stronger interfering signals are not present. Accordingly, neither of these techniques (CMA or power inversion) is suitable for MMSE weight generation in WLANs when interference is present.
U.S. Pat. No. 5,887,038 describes an adaptive array using a composite signal of weighted average of a coherent reference signal value and a constant modulus reference signal value. This technique also has problems distinguishing the desired signal from interfering signals.
It is desirable to provide an enhanced wireless communication system that employs multiple antenna elements and optimal adaptive signal processing techniques to provide an increase in operating range in a multipath environment while providing compatibility with existing wireless communication systems using blind weight adaptation.